I've got a workaround using stacked pictures as the fill - but this is very fiddly to set up.
I'd still be interested to learn of a better way to set the backcolor or forecolor to null/transparent - and end up with a column optionally with green, red or red and green stripes.

    
    With Selection.Format.Fill
        .Visible = msoTrue
        .Patterned msoPatternWideUpwardDiagonal
        .ForeColor.RGB = RGB(255, 0, 0)
        .BackColor.RGB = RGB(0, 255, 0)
    End With